The genus Monachus previously included three geographically widely separated species: the Mediterranean Monk Seal, Monachus monachus; the Caribbean Monk Seal, M. tropicalis; and the Hawaiian Monk Seal, M. schauinslandi (Rice 1998). Recent studies (Scheel et al. 2014) have shown that molecular, morphological, and temporal divergence between the Mediterranean and New World Monk Seals (Hawaiian and Caribbean) is equivalent to or greater than between sister genera of other phocids. As a result, Caribbean and Hawaiian Monk Seals have been reclassified together in a newly erected genus, Neomonachus. Considered the most primitive of all living phocid species, Monk Seals have anatomical features that resemble those of seal fossils from 14-16 million years ago (Barnes et al. 1985). Studies of N. schauinslandi DNA microsatellites revealed unprecedented low levels of diversity and heterozygosity. Evidence indicates low genetic diversity prior to recorded human influence, little contemporary inbreeding, and no significant population structure (Schultz et al. 2009, 2010).

The Hawaiian Monk Seal population was observed to decline from 1,520 total individuals in 1983 to 1,209 in 2011. The number of mature individuals in the entire population in 2011 was 632, a reduction of 20.4% from the 795 mature individuals in 1983. Currently the species is still in overall decline with two distinct trajectories between the populations in the NWHI (4.0% per year decline) and the MHI (6.5% per year increase; Caretta et al. 2013). The NWHI decline is mainly due to food limitation that could be due to changes in oceanographic conditions, legacy impacts of fisheries, or competition with other predators. The overall decline of more than 20% within two generations (30 years) meets IUCN criterion C1 for Endangered.

Hawaiian Monk Seals occur throughout the Hawaiian Island chain. Their six main reproductive sites are in the Northwestern Hawaiian Islands (NWHI) at Kure Atoll, Midway Atoll, Pearl and Hermes Reef, Lisianski Island, Laysan Island, and French Frigate Shoals (National Marine Fisheries Service 2007). While sightings were previously rare in the main Hawaiian Islands (MHI), Monk Seals are now regularly seen there and births have been documented on all of the major islands (Baker and Johanos 2004). Sightings outside of the main range have occurred at Johnston Atoll, Palmyra Atoll, and Wake Island (Ragen and Lavigne 1999).

Hawaiian Monk Seals are non-migratory, and tend to remain near the atoll where they were born. However, approximately 10% of seals will relocate temporarily or permanently to other sites in the island chain, and long distance wanderers have been recorded.

There are approximately 1,693 km of coastline habitat for Monk Seals in Hawaii (US Census Bureau 2012). However, the terrestrial habitat available for their use in the NWHI is very limited. The total area of emergent land is only about 13.5 km² and only a fraction of that is suitable for use by seals (Ragen and Lavigne 1999).

The extent of occurrence for Hawaiian Monk seals is approximately 1,500,000 km² and the area of occupancy is approximately 293,000 km².

Hawaiian Monk Seals are distributed throughout the Hawaiian Archipelago. Within the Monk Seal’s range, two regions are often distinguished: the NWHI, comprising eight subpopulations located on remote atolls and small islands, and the MHI, comprising eight large high islands and associated small islets. Seals in the MHI are considered to be a single subpopulation. The status of the Hawaiian Monk Seal is assessed annually and updated by the US National Marine Fisheries Service’s Pacific Islands Fisheries Science Center. The most recent reliable abundance estimates, from 2011, were used for this assessment.

The generation time for Hawaiian Monk Seals, estimated as the average age of reproducing individuals, is approximately 15 years. The best estimate of the total number of seals of all age classes in 2011 is 1,209. The population is comprised of approximately 632 sexually mature seals with the following distribution across subpopulations: Kure Atoll 58, Midway Atoll 27, Pearl and Hermes Reef 101, Lisianski Island 77, Laysan Island 113, French Frigate Shoals 117, Necker Island 38, Nihoa Island 43, and the MHI 58 (NMFS unpublished data).

The earliest information on Monk Seal abundance comes from 1958 (Kenyon and Rice 1959, Rice 1960). However, those data consisted of beach counts only and did not include an estimate of total population size. In order to compare total abundance over time, we have used an estimate for NWHI abundance in 1983 (Gerrodette 1985) at which time there were negligible numbers of seals known to exist in the MHI. This number is compared to the 2011 estimate for total abundance for both the NWHI and MHI. The total abundance estimate for 1983 was 1,520. In 2011, total range-wide abundance was estimated to be 1,209. Over this 28 year period the number of mature individuals declined by 20.4%, from 795 to 632. Extrapolating this decline pattern across two generations (30 years: 1984-2014) gives an estimated overall decline of 21.8%.

Overall, the Monk Seal population is still in decline, but with two distinct trajectories between the populations in the NWHI (4.0% per year decline) and the MHI (6.5% per year increase; Caretta et al. 2013). It is expected that species will continue to decline for at least the next 12 years if trends remain the same. At that point the number of animals in the MHI will surpass that in the NWHI and growth in the MHI will offset losses in the NWHI (NMFS unpublished data).

Adult Hawaiian Monk Seals reach lengths of about 2.1 to 2.4 m and weigh 170-240 kg. Pups are about 1 m and 16-18 kg at birth and when weaned six weeks later they weigh 50-100 kg (Kenyon 1981, Antonelis et al. 2003).

Female Monk Seals first give birth at 4-10 years old. Births occur during all months of the year, with most occurring during March to August (Johanos et al. 1994). Age specific fecundity rates for mature animals range from 50% to over 80%, and differ among subpopulations (Harting et al. 2007, Baker et al. 2011). Males in this polygynous species patrol the water adjacent to rookeries or land near females with pups. Male dominated sex ratios have occurred at some colonies, and that has resulted in mobbing of oestrus females some of which have been severely injured or killed in such events (Johanos et al. 2010).

Monk seals are generally solitary, both on land and at sea. Even when seals gather together on land, they are not normally gregarious and only mothers and pups and recently weaned seals regularly make physical contact. On land, Hawaiian Monk Seals haul-out and breed on substrates of sand, coral, or volcanic rock. Sandy beaches with shallow protected water near shore appear to be preferred for pupping (Westlake and Gilmartin 1990).

At-sea movements and habitat use of Hawaiian Monk Seals have been investigated using satellite-linked dive recorders and animal-borne video cameras. In the NWHI, Monk Seals forage within atolls, in the shallow waters surrounding atolls and islands, and farther offshore at submerged banks and reefs (Stewart et al. 2006). In the MHI, Monk Seals tend to stay within 50 km of shore and forage almost entirely within the 200 m bathymetric contour of the islands. Seals carrying cameras in both regions searched for and preyed on benthic fish and invertebrates in coral reefs, sand flats, as well as areas of rubble and consolidated bottom material, along areas of transition of benthic habitat types, and also in deepwater coral beds (Parish et al. 2000, 2002; NMFS unpublished data). Most dives that have been recorded have been less than 150 meters deep, although some individuals dived to more than 550 m (Stewart et al. 2006, NMFS unpublished data). Hawaiian Monk Seals are known to eat a variety of fishes, eels, cephalopods, and crustaceans (Goodman-Lowe 1998, Iverson et al. 2011, Cahoon et al. 2013).

Recovery of Hawaiian Monk Seals has been affected to an unknown degree by disturbance from military activities in the NWHI including the alteration, development, and occupation of bases on several key islands that started before World War II (Ragen and Lavigne 1999). However, the military no longer has bases in the area and the vast majority of Monk Seals live where they are isolated from most direct human contact. The only permanent structures remaining in the NWHI are US Fish and Wildlife Service facilities at Tern Island and Midway Atoll, and remnants of former Coast Guard facilities at Kure Atoll. Degradation of these structures, particularly at Tern Island, poses a significant entrapment hazard for Monk Seals. There are also concerns about legacy chemicals and contaminants that were left behind.

Current threats to Monk Seals are thoroughly reviewed and analysed in National Marine Fisheries Service (2007). The most crucial threats in the NWHI at this time are: 1) food limitation that could be due to changes in oceanographic conditions, legacy impacts of fisheries, or competition with other predators; 2) entanglement in marine debris, largely fragments of net and line discarded by North Pacific fisheries; 3) predation by sharks, especially on pre-weaned and recently weaned pups; and 4) intraspecific aggression by sub-dominant males. An emerging threat in this region may be the loss of terrestrial habitat due to sea level increases resulting from global warming (Baker et al. 2006). The situation in the MHI is different, with the main threats there being: 1) interactions with commercial and non-commercial fishing gear especially hookings and entanglements in gillnets; 2) possible transmission of diseases from feral animals, domestic pets, and livestock to seals; and 3) intentional killing and harassment.

The Hawaiian Monk Seal has been listed as endangered under the US Endangered Species Act since 1976. That law contains a number of provisions to protect the seals and their critical habitat. They are also covered by a general prohibition against unpermitted taking by the US Marine Mammal Protection Act of 1973.

Virtually all of the land and waters in the NWHI is included in one or more protected areas (the Northwestern Hawaiian Islands State Marine Refuge, the Kure Atoll State Wildlife Refuge, the Hawaiian Islands National Wildlife Refuge, the Midway Atoll National Wildlife Refuge, the Northwestern Hawaiian Islands Coral Reef Ecosystem Reserve, and the Papahanaumokuakea Marine National Monument) where human activities that could effect the seals or their habitats are either prohibited or strictly controlled.

Hawaiian Monk Seals are the focus of one of the most proactive marine mammal recovery programs in the world (Lowry et al. 2011). Historically, the primary focus of the species’ recovery program was to increase survival of juvenile and adult female seals in the NWHI. Important recovery efforts include: 1) cleaning up of marine debris and toxic chemicals; 2) minimizing human activities that could disturb seals hauled out on beaches; 3) mitigation of shark predation of young pups through limited shark removals or pup translocations; 4) removal and translocation of adult males to mitigate mortality associated with male aggression towards juveniles and females; and 5) regulating fisheries to reduce the likelihood of direct and indirect interactions. In spite of these efforts the population has continued to decline, however, the efforts have mitigated some of the severity of that decline. A recent analysis has shown that up to 30% of the current Monk Seal population is alive today due to the small-scale mitigation efforts targeted towards individual seals over the past 30 years (Harting et al. 2014).

Efforts to increase juvenile survival are also being expanded using a variety of translocation strategies. Over the past thirty years, numerous Monk Seal translocations have been conducted with a variety of objectives, including reducing shark predation and conspecific male aggression, stopping human–seal interactions, and taking advantage of different demographic patterns between subpopulations to improve survival. (Baker et al. 2011, 2014).

Management agencies are currently developing a vaccination plan to guard against a disease outbreak that could decimate the population. Two viruses, Morbillivirus and West Nile Virus, have been identified as infectious agents with a high risk to Hawaiian Monk Seals. Marine Morbillivirus strains have been recently identified in Hawaii and it considered only a matter of time before West Nile Virus makes it to Hawaii (Kilpatrick et al. 2004, West et al. 2013). Background surveys conducted on Hawaiian Monk Seals show that they remain naïve (no detectable antibodies) to both viruses. The vaccination program could begin as early as 2015.

In 2014, a privately funded and managed rehabilitation facility was constructed to support recovery efforts. This Hawaiian Monk Seal hospital will serve to care for seals from across the archipelago that are sick, injured, emaciated, or otherwise in need of rehabilitation.

With the increase of Monk Seals in the main Hawaiian Islands, the recovery program has had to develop a number of strategies to maintain population growth while mitigating human-seal interactions (Lowry et al. 2011). The program is currently working on strategies to prevent and modify unwanted seal behaviors including fisheries interactions and animals conditioned to interact with people. A significant component of this work includes community engagement on how to avoid negative wildlife interactions.